The invention in general relates to igniter bags, and in particular to igniter bags used to ignite stick propellant.
FIG. 1 schematically shows an example of the environment wherein a base pad igniter bag is used. A barrel 10 contains a projectile 12 that rests on a metal cartridge case 14. Case 14 is closed at its upper end by a polyurethane plug 16. Case 14 is filled with stick propellant 18. A case base 20 closes the lower end of the case 14. A stub igniter 22 is disposed centrally in the case base 20. A base pad igniter bag 24 fits in the cupped region of the case base 20.
Base pad igniter bag 24 is typically shaped like a doughnut to fit around stub igniter 22. Igniter bag 24 is generally sewn from clean-burning cloth and filled with a granular igniter material 26. Igniter bag 24 transfers the ignition from stub igniter 22 to the stick propellant 18. Initially, stub igniter 22 is ignited and it transfers its combustion products radially through openings 28 into the igniter bag 24. The combustion products from the igniter bag 24 vent into the stick propellant 18, igniting the ends of the sticks and flowing through the channels between the sticks to ignite their other surfaces.
FIG. 2 schematically shows a prior two-piece igniter bag 30 with a seam at the inner circumference 32. The bag of FIG. 2 is made by sewing two identically shaped pieces of material together along their inner and outer circumferences 32, 34. The design of FIG. 2 offers no real surface area normal to the venting gases from the stub igniter. This lack of intimate contact between the stub igniter and the igniter bag leads to excessive ignition delays. Providing more intimate contact is the object of the bag shown in FIG. 3.
FIG. 3 schematically shows a three-piece igniter bag 40 in which a separate flat piece 42 is sewn like a sleeve between the two doughnut shaped pieces of material. While better ignition is achieved, the bag of FIG. 3 takes much longer to make because of the difficult seam between the straight sleeve 42 and the two doughnut shaped main pieces. The small radius, on the order of two centimeters, is very extreme and difficult to sew by machine. In addition, the fabric bulk of the seam allowances is still in the critical region next to the stub igniter vents, so ignition transfer difficulties may be experienced.
The igniter bag of the present invention uses only two pieces of cloth, but retains the sleeve effect to achieve reliable ignition transfer to the igniter material contained in the bag. Seams are either straight or have a considerably larger radius of about 6 centimeters and are easier to sew. Furthermore, most seam allowance bulk is on the flat surface of the pad rather than near the interface with the stub igniter. Only two short, axial seams break the surface presented to the stub igniter.